This invention relates to a process for producing a rod lens array in which a multiple of gradient index rod lenses are precision arranged parallel to each other at given spacings and fixed between two side plates (frames) as they are buried in a resin. The rod lens array is typically useful as an optical component for an image writing system in an electrophoto-graphic printer.
The rod lens array is an optical component that has a multiple of tiny gradient index rod lenses arranged in alignment so that they combine together to form a single continuous erected unit-magnification image. The rod lens array has a short optical pathlength and needs no inverting mirror; because of the size-reducing feature, the rod lens array is commonly used as an optical component not only for an image reading system in an image scanner, a copier, etc. but also for an image writing system that forms a latent image on a photoreceptor in accordance with supplied image signals. Recent models of electrophotographic printer have been improved to achieve high image resolution comparable to that offered by silver halide photography. This has accordingly increased the requirement for higher precision in latent image and, hence, for better quality of image writing optical components in terms of the precision of the position in which image is formed.
A common process for the production of the rod lens array includes an assembly step in which a multiple of lens preformers are arranged in contact with each other and fixed between an upper and a lower side plate (frame plate) to form a block, an impregnation and curing step in which a resin is impregnated between the individual lens preformers in the block, and a cutting step in which the block is cut to a specified lens length. The side plates are usually fiber glass-reinforced plastic laminated plates (FRP plates).
In the assembly step, two methods are commonly used. In one method, a plurality of gradient index rod lenses are aligned on a flat frame such that their outer peripheral surfaces contact each other and they are altogether fixed to maintain the aligned state (this method is hereunder referred to as the xe2x80x9cdiameter referenced methodxe2x80x9d). The other method uses an aligning tool having a plurality of V-shaped grooves cut side by side in the surface of a platen on specified pitches, and gradient index rod lenses placed in the respective V-shaped grooves are fixed altogether to maintain their aligned state (this method is hereunder referred to as the xe2x80x9cmechanically referenced methodxe2x80x9d; see JP 9-90105 A).
The surface of an FRP plate has fine periodical asperities due to the texture of the woven glass fabric used in its production and hence the individual lens preformers are likely to be offset positionally during the assembly and impregnation steps. Positional off set also occurs due to the warpage of lens preformers and their surface roughness. In addition, both the diameter referenced method and the mechanically referenced method have their own problems.
Consider, for example, an image writing system of the type shown in FIG. 11; a light emitting portion (LED device) 10 blinks in accordance with image signals and a rod lens 12 forms a latent image on an imaging surface (photo receptor drum) 14. Any deviation from the desired alignment of rod lenses 12 results in a great variation in the potential at which the latent image is formed on the photoreceptor drum. On account of this variation in the imaging position that results from misalignment of the rod lenses, the image resolution that can be achieved by the image writing optical system is limited.
To make an acceptable rod lens array, gradient index rod lenses must be aligned such that adjacent lenses have a constant axial spacing (or xe2x80x9calignment pitchxe2x80x9d) and that there be neither inclination in the plane of alignment (which is hereunder referred to as xe2x80x9chorizontal inclinationxe2x80x9d) nor inclination in a direction normal to the plane of alignment (which is hereunder referred to as xe2x80x9cheight inclinationxe2x80x9d). The height inclination can be suppressed in the diameter referenced method. On the other hand, due to contact between gradient index rod lenses, a horizontally inclined lens affects an adjacent lens and the lenses taken as a whole may occasionally be inclined horizontally to suffer xe2x80x9caxial displacementxe2x80x9d. In the case of a printer or facsimile, this causes an image to be formed in a position far distant from where it should he.
The mechanically referenced method can provide higher precision in lens alignment. On the other hand, individual gradient index rod lenses sometimes fail to be placed uniformly within V-shaped grooves in the platen, causing one lens to be inclined with respect to another. Since it is unavoidable that lens preformers vary in diameter, the setting of the pitch between grooves in the platen must not be smaller than a maximum value for the variation in the diameter of lens preformers. As a result, very small gaps occur between arranged lens preformers and positional offset may occur during the mounting of side plates. In the xe2x80x9cpartialxe2x80x9d burial step where one side plate is mounted, the grooved platen adequately protects against positional offset but in the xe2x80x9ccompletexe2x80x9d burial step where the grooved platen is removed and the other side plate is mounted, the precision in the alignment of lens preformers may drop since there is nothing that controls the positional offset. In a printer or facsimile, this is a cause of failures such as an overlap of pixels.
Next, we describe the positional offset of rod lenses and their departure from the desired alignment on account of their surface roughness. The rod lenses to be used in the rod lens array are mainly produced by ion-exchange. As shown graphically in FIG. 17, a beam of incident light which falls on an end face of a rod lens 12 at an angle smaller than its angular aperture xcex80 is an effective ray 21 on the other hand, a beam of light incident at an angle greater than xcex80 undergoes regular reflection at the internal specular surface of the rod lens 12 which is manufactured by drawing. The reflected beam is so-called xe2x80x9cstrayxe2x80x9d light 22 which takes no part in image formation and therefore lowers the contrast of the rod lens 12. Furthermore, the rod lens array is constructed by a multiple of rod lenses 12 and the stray light 22 occurring in individual rod lenses 12 will reduce the overall contrast of the rod lens array.
In known rod lens arrays, a multiple of rod lenses arranged in one or two rows are fixed between two frame plates and a black silicone resin is filled between lenses and between each frame plate and lenses.
FIG. 18 shows a conventional method of removing stray light 22 by allowing it to scatter. To this end, the peripheral surface of the rod lens 12 is removed to some extent by a surface treatment and tiny asperities 23 are formed around it (see, for example, JP 58-38901 A). The stray light 22 incident on the surface of the area where asperities 23 are formed is scattered as indicated by 25. In addition, the black silicone resin 24 covering the peripheral surface of the rod lens 12 absorbs the scattered light 25, eventually suppressing the stray light 22.
In fact, however, the peripheral surface of the conventional rod lens has a profile as shown in FIG. 19 and suffers the following disadvantages. FIG. 19 shows the roughness, or the degree of unevenness, of the peripheral surface of a rectilinear area in the longitudinal direction of the conventional rod lens.
The first problem arises from the fact that the amount of removal from the peripheral lens surface varies from one lens to another and so does the lens diameter. If the rod lenses are arranged with reference to the frame plates, there occurs a departure from the desired lens arrangement due to the variation in lens diameter and the optical axis of one rod lens is inclined with respect to the optical axis of another.
Secondly, due to the lens-to-lens difference in the roughness of the peripheral lens surface, there occurs a variation in the effective aperture of the lens that contributes to satisfactory imaging. This causes a variation in the resolving power of the rod lens array in a longitudinal direction. With decreasing surface asperities, the lens surface has an increased degree of specularity and becomes more susceptible to the effect of stray light; it is therefore considered necessary that the peripheral surface of each rod lens have a center-line-average roughness of at least 0.5 xcexcm.
The present invention has been accomplished under these circumstances and has as an object providing a high-performance rod lens array that can be produced by the mechanically referenced method and which still features good axial alignment without suffering the inconsistency in the pitch on which gradient index rod lenses are arranged and two types of inclination, height and horizontal.
Another object of the invention is to provide a process for producing a rod lens array which not only prevents lens preformers from being positionally offset while they are arranged but which can also constrain their movement during installation of side plates, thereby suppressing the variation in the arrangement of lens preformers (their positional offset).
Another object of the invention is to provide a process for producing a rod lens array which can suitably be used as a component in an image writing optical system to insure higher precision in the imaging position.
Yet another object of the invention is to provide a process for producing a rod lens array using side plates furnished with a means for constraining the movement of lens preformers which can be easily formed in high dimensional precision and at low cost, allowing for the use of lens preformers or smaller diameter.
Further another object of the invention is to provide rod lenses that can be arranged with a small enough departure from the desired alignment to produce a rod lens array that will experience minimum variations in resolving power in a longitudinal direction.
The present inventors studied how the spacing between adjacent gradient index rod lenses to be used in producing a rod lens array by the mechanically referenced method would affect their alignment pitch of, as well as their height and horizontal inclinations. As a result, they found that by setting the spacing to lie within a specified range, the variation in the alignment pitch of rod lenses, the variation in their height inclination (standard deviation, hereunder referred to as xe2x80x9cheight variationxe2x80x9d) and the variation in their horizontal inclination (standard deviation, hereunder referred to as xe2x80x9chorizontal variationxe2x80x9d) could be reduced to their minimum levels. The present invention has been accomplished on the basis of this finding.
In order to attain the stated objects, the present invention provides the following two processes for production
(1) A process for producing a rod lens array comprising the steps of providing an aligning tool having a plurality of grooves formed side by side, placing gradient index rod lenses in alignment within the grooves at an average spacing of 1 xcexcm-5 xcexcm, fixing the gradient index rod lenses to form an integral unit as they maintain the aligned state, and thereafter polishing the end faces of each rod lens.
(2) A process for producing a rod lens array comprising the steps of providing an aligning tool having a plurality of grooves formed side by side, placing gradient index rod lenses in alignment within the grooves at an average spacing of 1 xcexcm-5 xcexcm, then fixing the gradient index rod lenses as they maintain the aligned state, thereafter removing the aligning tool, then cutting each rod lens to a specified lens length, and thereafter polishing the end faces of each rod lens.
To attain similar objects, the present invention provides a rod lens array that is produced by the process described above in (1) or (2) and which has the gradient index rod lenses aligned at an average spacing of 1 xcexcm-5 xcexcm.
The present invention provides a process for producing a single-row rod lens array comprising:
a lens preformer arranging step in which a grooved platen having a multiple of shallow grooves to receive gradient index rod lens preformers formed parallel to each other at equal spacings is supplied with a multiple of lens preformers such that they are placed in alignment within said shallow grooves;
a partial burial step in which an impregnating resin sheet and a side plate are placed in a face-to-face relationship with the group of arranged lens preformers, then heat is applied to render said resin sheet viscous and pressure is also applied so that the individual lens preformers are bonded to the side plate as they are partially buried in the resin and, thereafter, the bonded lens preformers ate detached from said platen; and
a complete burial step in which an impregnating resin sheet and a side plate are placed in a face-to-face relationship with the partially buried lens preformers in array form, then heat is applied to render said resin sheet viscous and pressure is also applied so that the individual lens preformers are bonded to the side plate as they are buried completely in the resin;
characterized in that a side plate with a striped pattern of multiple ridges that extend along the lens preformers are arranged on the same pitch as the lens preformers on the surface of a side plate substrate which is to bear the lens preformers is used as the side plate in the partial burial step or in both partial and complete burial steps and the lens preformers are bonded to said side plate such that each of them is located between adjacent ridges.
The present invention also provides a process for producing a two-row rod lens array comprising:.
a lens preformer arranging step in which a grooved platen having a multiple of shallow grooves to receive gradient index rod lens preformers formed parallel to each other at equal spacings is supplied with a multiple of lens preformers such that they are placed in alignment within said shallow grooves;
a partial burial step in which an impregnating resin sheet and a side plate are placed in a face-to-face relationship with the group of arranged lens preformers, then heat is applied to render said resin sheet viscous and pressure is also applied so that the individual lens preformers are bonded to the side plate as they are partially buried in the resin and, thereafter, the bonded lens preformers are detached from said platen, and
a complete burial step in which an impregnating resin sheet is placed between two arrays of the partially buried lens preformers such that the lens preformers in one array are in a face-to-face relationship with those in the other array, then heat is applied to render said resin sheet viscous and pressure is also applied so that the individual lens preformers are bonded to the side plate as they are buried completely in the resin;
characterized in that a side plate with a striped pattern of multiple ridges that extend along the lens preformers are arranged on the same pitch as the lens preformers on the surface of a side plate substrate which is to bear the lens preformers is used as the side plate in the partial burial step and the lens preformers are bonded to said side plate such that each of them is located between adjacent ridges.
In order to reduce the positional offset that may occur in imaging by the rod lens array, the variation in the arrangement of lens preformers has to be reduced. The variation in lens arrangement occurs not only when the lens preformers are arranged; it also occurs if lens preformers move during installation of side plates. In the present invention, the movement of lens preformers is constrained by the ridges which are formed on the side plates per se. Since these ridges are provided on the side plate substrate which is part of the final product per se, they have to be formed at low enough cost and, in addition, must satisfy high dimensional precision requirements so that lens preformers of smaller diameter (Dxe2x89xa61 mm) can be used. As a prior art technique, JP 7-46383 A has proposed that groove shapes be formed in the side plates of a lens array. To form groove shapes, grooves may be transferred to the side plates by pressing or they may be cut by mechanical working. However, realizing the depth and pitch of grooves that have sufficient precision to permit the use of finer rod lenses requires sophisticated facilities and high cost and, hence, is not practically feasible. In the present invention, a multiple of ridges rather than grooves are formed in high precision. Ridges are preferably formed by screen printing or photolithography. By either method, ridges of a rectangular cross section can be formed in precise alignment.
In the case of screen printing, a coating is desirably applied to the entire surface of one side of a side plate substrate to form an undercoat and a multiple of ridges to assist in arrangement of lens preformers are formed on the undercoat. The undercoat may be formed by applying one or two layers of a coat having a thickness of 5-15 xcexcm; the ridges may have a thickness of 10-30 xcexcm that is relatively easy to control in precision; the pitch of the ridges may be set at a value near the maximum variation in the diameter of lens preformers that is anticipated in the manufacturing process.
The side plate substrate may be a fiber glass-reinforced plastic laminated plate (FRP plate) or a glassplate. The undercoat is effective in reducing the tiny surface asperities that have been caused by the texture of the woven glass fabric. In the case of a glass plate, it is rendered light-opaque by mixing with a black pigment or the like so as to impart the ability to prevent leaky light (stray light). To insure adequate bonding strength at the interface with the resin to be impregnated between lens preformers, an epoxy resin based coating is desirably used to form the undercoat and ridges.
In the case of photolithography, a resist is applied to the entire surface of the side plate substrate which is then exposed to light through a mask and the areas that have become soluble are etched away to form a multiple of ridges in a desired pattern. Preferably, the conditions for resist application and etching are controlled such that ridges 10-30 xcexcm thick are formed on the underlying resist blanket having a thickness of 5-15 xcexcm.
The present invention provides a rod lens having a center-line-average roughness of 0.5 xcexcm-2.0 xcexcm on the peripheral surface.
It has been verified that if the peripheral surface of the rod lens has a center-line-average roughness of 0.5 xcexcm-2.0 xcexcm the effect of stray light can be eliminated to suppress the variation in resolving power.
The present invention also provides a rod lens array in which the constituent rod lenses are such that representative values for the center-line-average roughness on their peripheral surfaces are between 0.01 xcexcm and 0.2 xcexcm as averaged for the whole lens array.
This design has also been verified to be effective in eliminating the effect of stray light to suppress the variation in resolving power.
The present invention also provides a rod lens array in which the constituent rod lenses are such that representative values for the center-line-average roughness on their peripheral surfaces are between 0.01 xcexcm and 0.2 xcexcm as expressed by standard deviation for the whole lens array.
With this design, variation is less likely to occur in the effective lens aperture which contributes to image formation.
The present invention also provides a rod lens array in which the constituent rod lenses are such that representative values for their diameters are between 0.01 xcexcm and 2.5 xcexcm as expressed by standard deviation for the whole lens array.
With this design, departures from the desired lens arrangement are less likely to occur from the variation in rod lens diameter.
The representative values for the center-line-average roughness may each be a value on a straight line that extends on the peripheral surface of the lens parallel to its axis. Alternatively, they may each be the average of values on different straight lines that extend on the peripheral surface of the lens along its axis.
Each of the rod lens arrays described above has preferably a resin portion that is integral with the constituent rod lenses such that it fills the gap between adjacent rod lenses and surrounds all rod lenses.
In this design, the rod lenses are surrounded by the resin and stray light can be absorbed most effectively if the resin is a good light absorber such as a black resin.
Preferably, a frame plate is fixed to at least one of the two outer surfaces of said resin portion which are lateral to the thickness of the array.
With this design, a plurality of rod lenses can be easily arrayed with the frame plate used as a guide. As a result, a highly precise rod lens array can be realized.
The present disclosure relates to the subject matter contained in Japanese patent application Nos. 2000-298424 (filed on Sep. 29, 2000), 2000-343212 (filed on Nov. 10, 2000) and 2001-40110 (filed on Feb. 26, 2001), which are expressly incorporated herein by reference in their entireties.